1. Field of the Invention
The present invention relates to an electron emission element, a method for producing the same, and a light-emitting device using the same.
2. Description of the Related Art
In recent years, as an electron beam source for a flat display, an emitter portion of a vacuum device that can be operated at high speed and the like, a cold cathode electron source has been considered, which replaces a hot cathode electron source requiring heating. There are various types of cold cathode electron sources. In particular, a field emission (FE)-type, a tunnel injection (MIM, MIS)-type, and a surface conduction (SC)-type are known.
In a FE-type electron source, an electric field is applied to a cone-shaped projection (electron emission portion) made of silicon (Si), molybdenum (Mo), or the like, whereby electrons are emitted from the end of the projection. In MIM-type and MIS-type electron sources, a layered structure (e.g., metal/insulator/metal (or semiconductor)) is formed, and electrons are injected through the metal side, whereby the injected electrons are taken out of an electron emission portion. Furthermore, in an SC-type electron source, an electric current is allowed, to flow in an in-plane direction of a thin film formed on a substrate, whereby conductive electrons are partially taken out of a previously formed crack portion in the thin film.
The above-mentioned elements have features in that they can be minimized and integrated by using fine processing technology. These elements also have features in that heating is not required, unlike a hot cathode electron source.
FIG. 14 shows an example of a conventional electron emission element using a FE-type electron source. Referring to FIG. 14, a conventional electron emission element 1 includes a substrate 2, a cathode electrode 3 formed on the substrate 2, a cone-shaped electron emission member 4 disposed on the cathode electrode 3, an anode electrode 5 disposed so as to be opposed to the cathode electrode 3, a control electrode 6 disposed between the cathode electrode 3 and the anode electrode 5, and an insulating layer 7 supporting the control electrode 6.
In general, the following characteristics are desired for an electron emission material and an electron emission element using the same. (1) Electrons can be emitted at a low electric power (i.e., a material has a high electron emission ability). (2) Stable electron emission characteristics can be maintained (i.e., an emitter portion is chemically/physically stable). (3) Outstanding wear resistance and heat resistance can be obtained.
However, in the conventional electron emission element 1, an emission amount of electrons greatly depends upon the shape of the electron emission member 4, and it is very difficult to produce and control the electron emission member 4. Furthermore, Si, Mo, and the like generally used as a material for the electron emission member 4; do not have sufficient surface stability.
Therefore, conventionally, an electron emission material for use in an electron emission member has been studied. In particular, carbon materials have been considered as those which are capable of emitting electrons even at a low electric field. For example, it is reported that carbon fiber functions as a field emitter irrespective of its relatively high work function. Furthermore, it is reported that a carbon nanotube has a six-membered net of carbon wound in a cylindrical shape, and electrons are likely to be emitted from an end facet thereof However, carbon materials such as a carbon nanotube are difficult to handle as an electron emission material due to their powdery shape and brittleness. It is also difficult to dispose carbon nanotubes in such a manner as to control the direction of the end facets thereof, which are likely to emit electrons.
As described above, the electron emission elements and electron emission materials that have been used do not sufficiently satisfy the required characteristics and are difficult to handle.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an electron emission element having a high electron emission ability, a method for producing the same; and a light-emitting device using the same.
In order to achieve the above-mentioned objective, an electron emission element of the present invention includes: a cathode electrode; an anode electrode disposed so as to be opposed to the cathode electrode; and an electron emission member disposed on the cathode electrode, wherein the electron emission member includes a first member having a hole and a second member filling the hole, and the second member is more likely to emit electrons than the first member. In this electron emission element, a material that has a high electron emission ability but is difficult to handle can be used as a material for the second member. Therefore, an electron emission element with a high electron emission ability can be obtained.
It is preferable that the above-mentioned electron emission element further includes a control electrode between the anode electrode and the cathode electrode, for controlling electron emission from the electron emission member. According to this structure, an electron emission amount from the electron emission member can be controlled easily by changing an electric potential of the control electrode.
In the above-mentioned electron emission element, it is preferable that the first member includes an insulating layer on an outer periphery, and the control electrode is formed on the insulating layer. According to this structure, the control electrode can be disposed with good precision, so that electron emission from the second member can easily be controlled.
In the above-mentioned electron emission element, the hole is preferably a through-hole. According to this structure, the first member can be filled easily with the second member, so that an electron emission element that can be produced easily, is obtained.
In the above-mentioned electron emission element, it is preferable that the electron emission member includes a convex portion formed of the second member on its end portion on the anode electrode side. According to this structure, an electric field can be concentrated on the convex portion of the second member, so that an electron emission element with a particularly high electron emission ability can be obtained.
In the above-mentioned electron emission element, it is preferable that the second member contains an allotrope of carbon (C). Due to this structure, an electron emission element with a particularly high electron emission ability can be obtained.
In the above-mentioned electron emission element, it is preferable that the second member contains an allotrope of carbon having a graphene structure. Furthermore, it is particularly preferable that the allotrope is a carbon nanotube. Furthermore, it is particularly preferable that a content of the carbon nanotube in the second member is 1% by volume or more. With this structure, an electron emission element with a particularly high electron emission ability is obtained.
In the above-mentioned electron emission element, it is preferable that the second member further includes at least one selected from the group consisting of graphite, fullerene, diamond and diamond-like carbon. With this structure, a highly stable electron emission element is obtained.
In the above-mentioned electron emission element, it is preferable that the first member is made of metal. According to this structure, since metal is easily processed, an electron emission element that can be easily produced is obtained. In particular, it is preferable in terms of safety that the first member contains at least one metal which does not react with carbon, selected from the group consisting of Au, Ag, Cu, Pt, and Al.
In the above-mentioned electron emission element, it is preferable that the first member has a cylindrical shape, and the second member has a columnar shape. According to this structure, an electron emission element that can be easily produced is obtained.
According to the present invention, there is provided a method for producing an electron emission element including a cathode electrode, an anode electrode disposed so as to be opposed to the cathode electrode, and an electron emission member disposed on the cathode electrode. The method includes: filling a substantially cylindrical body made of a first material with a second material different from the first material, followed by drawing and cutting, thereby forming an electron emission member including a first member with a through-hole and a second member that fills the through-hole and is more likely to emit electrons than the first member. According this production method, an electron emission element having a high electron emission ability can be easily produced.
In the above-mentioned production method, it is preferable that the second material contains a carbon nanotube. According to this structure, carbon nanotubes can be arranged substantially in one direction during drawing. Therefore, an electron emission element with a particularly high electron emission ability can be easily produced.
It is preferable that the above-mentioned production method further includes: removing an end portion of the first member to form a convex portion formed of the second member on the electron emission member after forming the electron emission member. According to this structure, an electron emission element with a particularly high electron emission ability can be produced easily.
A light-emitting device of the present invention includes: a substantially vacuum container and a plurality of electron emission elements disposed in the container, wherein the electron emission elements are those of the present invention, and a phosphor film is disposed between the electron emission member and the anode electrode. In this light-emitting device, a light-emitting device with a high light emission intensity is obtained.
It is preferable that the above-mentioned light-emitting device further includes a control electrode between the anode electrode and the cathode electrode, for controlling electron emission from the electron emission member.
In the above-mentioned light-emitting device, it is preferable that the first member includes an insulating layer on an outer periphery, and the control electrode is formed on the insulating layer.
In the above-mentioned light-emitting device, it is preferable that each control electrode of the plurality of electron emission elements is independently controlled. Due to this structure, an image output device with a high light emission intensity is obtained.
In the above-mentioned light-emitting device, it is preferable that the hole is a through-hole.
In the above-mentioned light-emitting device, the electron emission member includes a convex portion formed of the second member on its end portion on the anode electrode side.
In the above-mentioned light-emitting device, it is preferable that the second member contains an allotrope of carbon.
In the above-mentioned light-emitting device, it is preferable that the second member contains an allotrope of carbon with a graphene structure.
In the above-mentioned light-emitting device, the allotrope is a carbon nanotube.
In the above-mentioned light-emitting device, it is preferable that a content of the carbon nanotube in the second member is 1% by volume or more.
In the above-mentioned light-emitting device, it is preferable that the second member further includes at least one selected from the group consisting of graphite, fullerene, diamond and diamond-like carbon.
In the above-mentioned light-emitting device, it is preferable that the first member is made of metal. In particular, it is preferable that the first member contains at least one metal which does not react with carbon, selected from the group consisting of Au, Ag, Cu, Pt, and Al.
In the light-emitting device, it is preferable that the first member has a cylindrical shape, and the second member has a columnar shape.